Recently, with the development of wireless communication technology, onboard information equipment typified by a car navigation system has offered user applications that adopt various wireless technology. As an instance, there are a lot of examples such as payment of a fee using an ETC (Electronic Toll Collection System: a registered trademark, the mention of which will be omitted from now on), an in-vehicle wireless communication system for connecting between user equipment like a Bluetooth/wireless LAN and onboard information equipment or between two or more pieces of onboard information equipment, an outside-vehicle wireless communication system employing mobile phone/wireless LAN (Local Area Network), and a dedicated onboard wireless audio transmission system. In addition, a rate of installation of user applications adopting the wireless technology into onboard information equipment is increasing.
When different schemes with nearby frequencies are mixed in a wireless communication system, communication deterioration can occur because of radio interference, that is, because radio waves used in different wireless systems each appear as noise. In particular, a wireless LAN or Bluetooth (registered trademark: it is also applied to the drawings, the mention of which will be omitted from now on) which is incorporated into a lot of products such as a mobile phone, notebook computer and game machine employs a frequency channel of 2.4 GHz band.
FIG. 11 shows a frequency channel arrangement of a wireless LAN and Bluetooth. These frequency channel arrangements are defined in reference 1 and reference 2 shown below, and frequency channels used are determined in accordance with operations in each country. The 2.4 GHz band, which is called ISM (Industrial, Scientific and Medical) band, is under lenient operation regulations compared with the other frequency band and does not require a radio station license. Accordingly, it is widely used by a small power wireless data communication system, and is used for a household microwave oven or medical equipment in addition to communication.
Reference 1: IEEE-Std. 802.11™-2007 IEEE Standard for Information technology—Telecommunications and information exchange between systems—Local and metropolitan area networks—Specific requirements Part 11: Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY) Specifications.
Reference 2: BLUETOOTH SPECIFICATION Version 3.0+HS.
A wireless LAN, Bluetooth and other wireless communication schemes used in the 2.4 GHz band have their own unique interference avoidance functions. For example, the wireless LAN, which has a CSMA/CA (Carrier Sense Multiple Access with Collision Avoidance) function, decides on whether the frequency band it uses is occupied by any other wireless device by actually measuring a used state of the radio wave, and avoids an interference or collision with the other wireless LAN device or other wireless device with other wireless communication scheme by carrying out communication when it decides that no other wireless device uses the frequency band. In a domain where the frequency band it uses is occupied by the other wireless LAN device or wireless device with other wireless communication scheme, since the ratio of being able to occupy it for its own communication reduces, the transmission efficiency of the communication reduces.
As for the Bluetooth, although it carries out spread spectrum communication by executing frequency hopping, it avoids the interference by measuring the communication state of each hopping frequency using the AFH (Adaptive Frequency Hopping) function, and by removing a frequency with a bad communication state due to radio interference or the like from the hopping frequencies to prevent using it. However, since the number of the hopping frequencies has a lowest limit, when a large percentage of the frequency band used undergoes interference, the communication deterioration increases.
In addition, there are many devices recently that incorporate a wireless LAN, Bluetooth and other scheme, which employ the 2.4 GHz band, into a single wireless device. When the plurality of wireless communication schemes installed in the single wireless device are used simultaneously, since the frequency band is the same, unless the configuration is made in such a manner that the frequency channel assignment is made appropriately or time-division multiplexing control is carried out when the frequency channels are used in common, deterioration due to radio interference can occur.
As an interference avoidance method of a wireless device that incorporates a plurality of wireless communication schemes, Patent Document 1, for example, discloses, in a composite wireless device that incorporates both the Bluetooth and wireless LAN, a configuration that prevents the hopping frequencies of the Bluetooth from encountering the carrier sense of the wireless LAN by controlling in such a manner that the hopping frequencies of the Bluetooth are not allocated to the frequency channel used by the wireless LAN and to its neighboring frequencies. Thus, it avoids the interference between the wireless devices within the composite wireless device which includes multiple wireless communication schemes.
In addition, Patent Document 2 discloses, in an onboard communication device that incorporates both the Bluetooth and wireless LAN, an onboard communication device which comprises an in-vehicle communication means for connecting an in-vehicle mobile terminal and the Bluetooth, and an outside-vehicle communication means for connecting a wireless LAN base station outside the vehicle with the wireless LAN, and which optimizes, when an occurrence of wireless communication by the outside-vehicle communication means is expected, the frequency the in-vehicle communication means will use in the wireless communication so as not to include the particular frequency the outside-vehicle communication means uses.